I'm going to do a 2 part series about the the different sizes and types of wrenches grips find themselves using.
To start things off, the mother of all grip wrenches, the one that makes almost everything we do possible:
3/16" hex key
The majority of the fittings we use on speedrail and 5/8" rail, and a lot of other stuff uses 3/8" set screws or "grub nuts" that use a 3/16" hex or allen key. There are many different types of hex wrenches out there and they all have different strengths and weaknesses. I haven't met a style of wrench that didn't have it's merits in one situation or another.
But far and away the most common, mostly useful type of 3/16" hex wrench is the speed wrench. This compact little guy doesn't take up much space on your belt and is long enough to get you the leverage you need. Another popular option is getting a stubby ratchet and then put a hex key socket on it. With that setup, you can also put a rubber band on the socket to get a little thumb wheel action going on. You can also get a set of allen keys which is handy for when you need to fix a riser on a stand as they usually use a different size. Speaking of which, the other hex key size that is pretty common is 7/32". That is the size used in countersunk bolts for cheeseplates.